Lead-free Research And Performance Optimization Of Barium Titanate Positive Temperature Coefficient Resistance

Lead-free research of barium titanate positive temperature coefficient (abbreviated as PTC) resistance was conducted in this dissertation, and simultaneously elements’ parameters improvements without the Curie temperature (abbreviated as Tc) were obtained. As a response of environment protection, the purpose of our research is to search for appropriate substitute of lead, to decrease its damage to the environment and human and to enhance element performance. During this process, we made great efforts to obtain a deep recognition of element microscopic mechanism and to propose some novel opinions. Main achievements are listed as follows.(1) Combined with the theory model of barium titanate positive temperature coefficient resistance, a heat treatment method has been developed to reduce the oxygen concentration in grain boundaries for decrease in element room temperature resistivity. By certain resistance-reducing reagent, resistivity can be reduced effectively in elements with various resistivities. In one hand, this method can promote the recycling of element with huge resistivity for considerable economic value. In another hand, this method makes us have a better understanding of grain boundary model in barium titanate elements. Basing on experiment results, we proposed a semi-quantitative boundary model. It can explain the effect of oxygen concentration change on element temperature-resistance property and reveals the compensation degree of spontaneous polarization to acceptor state in grain boundary.In addition, combined with analysis of other acceptors’role, the model can provide a better judgment about the acceptor introduction degree by additives in synthesis of lead-free element. The method above has been effectively used in related enterprise and an invention patent of the method has been authorized.(2) By solid phase method, lead-free elements with addition of K0.5Bi0.5TiO3 (abbreviated as KBT) have been synthesized. Using different technology methods, practical lead-free elements have been produced by adding KBT or adding K2CO3, Bi2O3 and TiO2 separately. The elements exhibit higher Tc (around 135℃) than barium titanate and ultra low room temperature resistivity (13.84Ω·cm). This result shows the problem that KBT usually increase element room temperature resistance greatly can be solved, which provide a good foundation for more KBT addition.In this experiment process, we proposed a sintering schedule which contained a heat preservation stage around melt point of K2O and Bi2O3. It enhanced Tc and reduced room temperature resistivity and enriched our recognition to thermodynamics in sintering process.(3) By solid phase method, elements with Tc of 200℃ have been produced with a proper addition ratio of KBT and lead. Compared to elements containing lead only, lead concentration in element has been decreased by 50%. This method inhibits the usage of lead in element and realizes partial substitution of lead in high temperature heating element.(4) Elements with different Tc have been produced by experiments which have added Ca、 Y、Na0.5Bi0.5TiO3 (abbreviated as NBT) and KBT in barium titanate, respectively. Through comprehensive analysis of the results, we found that stress in grain was a negative factor for increase of Tc. Conclusions can be made that grain boundary structure and stress distribution are important factors to affect the element performance and that synthesis technology improvement is necessary.(5) By adding K2CO3, Bi2O3 and TiO2 separately, elements’ resistance-temperature coefficient has been enhanced to 53%/℃ from 16%/℃, so element sensitivity has obtained a great increase. In our opinion, residual K in grain boundary has increased the surface acceptor state concentration for the increment of resistance-temperature coefficient. Moreover, in sintering process, the volatilization of K and Bi should been serious so element Tc did not change in this experiment. This also reveals that whether K and Bi have a mole ratio of 1:1 is important for increment of Tc in microscopic local areas.(6) Basing on theory about cooling and precipitation of melting crystal, we propose a sintering technology that adding a cooling stage around melting point of K2CO3 and Bi2O3. This technology has enhance the semiconducting rate effectively and element room temperature has decreased from 514.55Ω·cm to 112.95Ω·cm (with a proportion of 80%).According to the theory that precipitation is more effective in position with more defects, the technology reveals a novel synthesis method in ceramic sintering. Considering liquid phase sintering theories, it can be transplanted to other ceramic synthesis process with certain liquid phase.Basing on achievements above, we have a deeper recognition of barium titanate element. Some key problems in lead-free research are listed as follows.(1) The requirement of K and Bi have a mole ratio of 1:1 is not just in the degree of formula, but also in the degree of microstructure. This ratio is very important to enhance Tc of element.(2) Usually the substitutes of lead own a different lattice parameter to barium titanate and stress produced from this is a negative factor for higher Tc. To inhibit the stress and keep a relative low resistivity are key points to fabricate practical elements.(3) Element property is easily affected by a few grain boundaries because PTC effect originates from boundaries with low coherence. In another words, element’s electrical property is sensitive to microscopic structure and composition. Under a comprehensive consideration about the characteristic of different additives, design and exploitation of new manufacture technology is a key approach to remove lead from elements.In a word, achievements and experience of this dissertation have relatively good reference value to other researchers and have overcome some common difficulties in lead-free research. In our opinion, lead-free research has a good prospect with the development of material science.